The influence of social stress on cardiovascular (CV) response in laboratory rats will be investigated in two main studies including four experiments. The primary objective will be to thoroughly characterize the time course of response including systolic blood pressure, diastolic blood pressure, mean arterial pressure, and heart rate, following exposure to social stress. Social stress will be in the form of repeated exposure to defeat by a larger conspecific rat, and its significance as an important CV stressor will be examined by comparing CV response to defeat with CV response to other frequently studied stressors (electric foot- shock, and forced immobilization). CV response will be recorded before, during and continuoulsy after exposure to repeated defeats (or other stressors). The importance of genotypic influences on CV responsivity to stress will also be examined by testing three different inbred strains of laboratory rats, two of which (the salt-sensitive S/JR and salt-resistant R/JR rats) were selected for differential susceptibility to hypertenion. Experiemnts will also investigate the critical aspects of the defeat experience which contribute to alteration of CV response. The stimuli associated with defeat will be manipulated, the time between defeats will be systemically controlled, and the duration of the defeat experience will be controlled, in order to identify the specific social mechanisms affecting CV function. It is anticipated that these experiments provide data which will be instrumental in designing further research which directly addresses the mechanisms controlling CV response to social stress.